In this chapter, we looked at more advanced ways to gather data.
We explored log monitoring and either tracking a single file or multiple files and matching a regular expression. We filtered the results and parsed some values out of them.
Calculated items gave us a field to type any custom formula and the results were computed from the data the server already had, without querying the monitored devices again. Any trigger function could be used, providing great flexibility.
Aggregate items allowed us to calculate particular values, such as the minimum, maximum, and average for items over a host group. This method is mostly useful for cluster or cluster-like systems, where hosts in the group are working to provide a common service.
External checks and user parameters provided a way to retrieve nearly any value—at least any that can be obtained on the command line...